/Engine/Source/Runtime/Engine/Classes/Animation/AnimMontage.h
A montage can only have a single leader. A leader can have multiple followers. Loops cause no harm. If Follower gets ticked before Leader, then synchronization will be performed with a frame of lag. Essentially correcting the previous frame. Which is enough for simple cases (i.e. no timeline jumps from notifies). If Follower gets ticked after Leader, then synchronization will be exact and support more complex cases (i.e. timeline jumps). This can be enforced by setting up tick pre-requisites if desired.
void MontageSync_Follow ( struct [FAnimMontageInstance](API\Runtime\Engine\Animation\FAnimMontageInstance) * NewLeaderMontageInstance )